Very little difference. At stock speed the the i5-3570k is 100MHz faster. However, Ivy Bridge CPUs are also about 5% faster (efficient) than Sandy Bridge CPUs so the i5-3570k would be equivalent to an i5-2500k running at 3.57GHz; a difference of only 270MHz.
Due to heat issues, IB CPUs do not overclock as well as SB CPUs. I think the general upper limit is 4.5GHz for IB. But that is equivalent to 4.73GHz for SB, and SB CPUs are generally limited to 4.8GHz, but I've seen them go as high as 5.0GHz for short periods of time.
IB CPUs generally consume around 15w - 20w less power than SB CPUs. Not a big difference.
IB CPUs support PCI-e 3.0 while SB CPUs supports up to PCI-e 2.0. For current generation graphic cards and the probably the next two generations, it's not going to make a big difference. But beyond that the really high end graphic cards will likely start to be limited by PCI-e 2.0. That would be the hypothetical Radeon HD 10950 / 10970 / 10990 and the GTX 970 / 980 / 990 GPUs.